Automatic shifting power transmissions, whether of the planetary or countershaft type, utilizing hydraulically actuated friction clutches and brakes incorporate shift valves for controlling the interchange and engagement of each of the friction devices. These shift valves have been of various designs including fully hydraulically actuated valves and fully solenoid controlled valves with a combination of solenoid and hydraulic actuation.
It has also been desirable in power transmissions when utilizing electrical members to ensure that the power transmission will remain in at least one selected gear should the electrical energy be discontinued between the source and the control valves.